In acoustic wave devices, an Interdigital Transducer (IDT) exciting an acoustic wave is formed on a piezoelectric substrate. The piezoelectric substrate is, for example, a lithium tantalate (LiTaO3) substrate or a lithium niobate (LiNbO3) substrate. When the Li compositions in lithium tantalate and lithium niobate are stoichiometric, they are called a stoichiometry composition. When the Li composition is a little less than the stoichiometric composition, it is called a congruent composition. Most of the lithium tantalate substrates and the lithium niobate substrates have a congruent composition.
Japanese Patent Application Publication No. 2013-66032 (Patent Document 1) describes that lithium is diffused to the surface of a substrate with a congruent composition to form a region with a stoichiometry composition on the substrate surface. Japanese Patent Application Publication Nos. 2011-135245 and 2002-305426 (Patent Documents 2 and 3) describe that a piezoelectric substance with a stoichiometry composition is used for an acoustic wave device. International Publication No. 2013/031651 (Patent Document 4) describes that a dielectric film is provided under a piezoelectric film.
To reduce the loss of the acoustic wave device, it is required to reduce the leak of the acoustic wave excited by the IDT. However, there is no known preferable structure in the piezoelectric substrate for reducing the loss of the acoustic wave device.